This invention relates to a vibrator and an ultrasonic motor employing the same, and more particularly to a vibrator for exciting substantially elliptical motion and an ultrasonic motor employing such a vibrator.
Conventionally, a standing-wave-type ultrasonic motor has been mainly used to achieve a high efficient vibration and a high output power. The conventional standing-wave-type ultrasonic motor operates by holding a moving element under a prescribed pressure against a vibrator which excites substantially elliptical motion and driving the moving element under frictional forces between the moving element and material points of a stator which make substantially elliptical motion.
As a conventional vibrator used in the conventional ultrasonic motor for exciting the substantially elliptical motion, a vibrator employing an elastic member having a particular shape, such as a cantilever type vibrator, a planar vibrator and the like, is used and various vibration modes such as shearing vibration, flexural vibration or the like is adopted. Further, in order to excite the substantially elliptical motion at a high efficiency, the vibrator employs a mechanical resonant system having a natural vibration mode which effects unidirectional vibration at the contact surface between the elastic member and an exciting member and which effects elliptical vibration at the contact surface between the elastic member and the moving member. The former examples are described in Japanese Laid-open Patent Publication Nos. 61-129077 and 62-262676, in Japanese Patent Application No. 63-97152 and in a collection of papers (A224, 225) of "ELECTRICAL INFORMATION COMMUNICATION LEARNED SOCIETY" in 1988, and the latter examples are described in Japanese Patent Application Nos. 63-97151 and 63-97152.
However, the above conventional vibrators have the following disadvantages.
In the cantilever type vibrator as shown in the Japanese Laid-Open Patent Publication 61-129077, it is difficult to completely fix an equilibrium point of vibration because the equilibrium point of the vibration is positioned at an end portion of a resonant member, so that the vibrator can not excite the substantially elliptical motion at high efficiency.
In the planar vibrator in which shearing vibration is excited in a planar resonant member, it is difficult to excite uniform shearing vibration to the vibrator because the boundary conditions at a fixed end portion of the planar resonant member are unstable and a strain occurs at a portion of the planar resonant member, so that vibration having large amplitude is not obtained at high efficiency.
In Japanese Laid-open Patent Publication No. 62-262676, only one type of standing wave is used and therefore the control of the moving direction is not performed by only one motor.
In one of the above papers, "A SHEET FEEDING DEVICE (I) AND (II) USING A PLANAR PIEZOELECTRIC VIBRATOR" disclosing an ultrasonic motor in which a surface-vertically bending vibration mode is used, a torsional strain is applied to an piezoelectric element, and therefore the vibration of this ultrasonic motor is suppressed and an excitation efficiency becomes low.
In Japanese Patent Application Nos. 63-97151 and 63-97152, since strains are concentrated on a fixing portion at which an ultrasonic vibrator is fixed, it is difficult to perform a complete fixation and effect impedance matching. In addition, the boundary conditions at the fixing portion tend to become unstable, so that the ultrasonic motors of these prior arts do not operate stably. Further, since a vibration profile of an elastic member is changed by the fixing portion, a desired vibration is not necessarily obtained and a high efficient operation is difficult. Accordingly, the ultrasonic motor having the ultrasonic vibrator of these prior arts can not provide kinetic energy (output power) at high efficiency.